Figure out a way to version published documentation.

[nfachan]

There are at least two reasons I want this.

First, it could be useful for a user to go back to an older version of the documentation if they're running an older version of the software. They may have a reason for not updating their software.

Second, when new features are added, or changes are made, we'd like to update the documentation concurrent with the changes. But if that immediately publishes to the public documentation, users are going to be confused because the documentation doesn't match their software.

At the very least, I want to come up with a solution to the second problem. If we're just solving this problem, we may just want to not publish the documentation automatically in CI, and instead have publishing it be a part of releasing.

[nfachan]

Okay, we now have this.

In doc/book there are subdirectories, one for each version, plus a symlink for latest.